No one could reasonably have expected "The Brady Bunch Variety Hour," which premiered Sunday on ABC, to be anything finer than insipid. Even in television, it makes no sense to build a musical show around nonmusical people.

That would scarcely seem to be the problem for "The Jacksons," the new CBS half-hour musical-variety series premiering tonight at 8:30 on Channel 9. And yet despite the proven pop prowess of the group - which has sold 60 million records first as the Jackson Five, now as the Jacksons - the hastily arranged show proves listless and slipshod.

Perhaps it will get better. There doesn't seem to be much choice.

Part of the problem for producer-director Bill Davis and the talented Jackson family is that this is a short-notice series, hurriedly thrown together when Norman Learn's "A Year at the Top" fell apart and was scrubbed on the launch pad. Still, one would think that five Hollywood writers would be able to come up with better comedy material to fill the gaps between numbers than the staff handed the Jacksons. By comparison, the Captain and Tennille are hilarious and Sonny and Cher positively Shavian.

Somehow, though, even the songs, which include the recent hit "Enjoy Yourself," don't really register either, partly because canned audience hysteria has been ladled over them and partly because the Jackson seem a bit surprised at being on television, even after three previous attempts.

One of those was a well-received CBS summer series last year. Another was an animated version still in syndication. Sadly, the real-life Jacksons on CBS tonight come off just about as dimensional as their cartoon counterparts did. "It's been great fun," says Michael Jackson, 17, at the show's close, but he could only be speaking for himself. "The Jacksons' have a long way to go before it is either an approximation of fun or an intimation of great.